Combined cycle power plants are known in the art as an efficient means for converting fossil fuels to thermal, mechanical and/or electrical energy. Such systems are described in U.S. Pat. No. 4,932,204 dated Jun. 12, 1990; U.S. Pat. No. 5,255,505 dated Oct. 26, 1993; U.S. Pat. No. 5,357,746 dated Oct. 25, 1994; U.S. Pat. No. 5,431,007 dated Jul. 11, 1995; and U.S. Pat. No. 5,697,208 dated Dec. 16, 1997; each of which is incorporated by reference herein.
It is known in the art to use air from the outlet of the compressor section of a gas turbine system to cool selected turbine parts, and further, it is known to cool the compressed air after it leaves the compressor and before reintroducing it into the turbine. Typical prior art methods for cooling this air are discussed in the above mentioned U.S. Pat. No. 5,697,208. These include using a fin/fan heat exchanger that would discharge the removed heat into the atmosphere as waste, or using this energy to pre-heat fuel for the gas turbine. As the compression ratios of compressors have increased, the temperature of the compressed air produced by the compressor has increased. At the same time, the cooling requirements for the hot turbine parts has increased due to increased firing temperatures. Most recently, it has become known in the art to cool this compressed air by passing it through a once-through cooler, and using the heat to generate high pressure steam. However, such prior art systems do not provide optimal levels of cooling for combined cycle power plants utilizing the most modern engine designs.
Due to high firing temperatures and the need to design higher efficiency combustion turbines, efficient methods for cooling hot components with the combustion turbine have been developed. One particular cooling scheme that has been developed passes steam through very small cooling passages in various parts of the turbine. These passages may be subject to blockage if the cooling steam is not maintained at a very high purity level. Furthermore, exotic alloys are being developed and used for these higher temperature applications. These materials may be subject to degradation if the cooling steam is not very pure. The source of cooling steam in prior art applications is often the intermediate pressure steam produced in the heat recovery steam generator. With a traditional blowdown scheme and for the pressure range in which the intermediate pressure evaporator may typically operate, the American Boiler Manufacturers Association (ABMA) recommends a maximum concentration of total dissolved solids (TDS) of about 2,500 ppm within the drum. The maximum fractional carryover recommended by the ABMA for this typical pressure is 0.0005. This corresponds to a steam TDS of about 1 ppm which is unacceptable for some new steam cooled combined cycle plant applications. Prior to this invention, the steam purity has been improved by improving the quality of the incoming feedwater to maintain the concentration of impurities in the drum to low levels. This is done by using condensate polishing systems. Such systems have proven to be expensive and unable to provide the desired steam quality.
There is also an ongoing need to reduce the boiler blowdown flow from combined cycle plants. Waste water is both difficult to dispose of and expensive to replace as makeup to the cycle. As such, it is advantageous to offer a power plant design which has the lowest level of boiler blowdown flow.
The market continues to demand increasing efficiency from combined cycle power plant designs. Modern advanced turbine systems have plant efficiency goals of 60% and more. To achieve such levels of performance, system designs must incorporate even higher compression ratios and higher combustion temperatures, as well as advanced cooling techniques with new exotic metals capable of withstanding such operating conditions. Furthermore, system designs which waste heat to the environment are no longer favored for both environmental and efficiency reasons.